This invention relates generally to a method and apparatus for facilitating both the visual and physical examination of the inside of a relatively large pressure vessel; more specifically, it relates to the temporary installation of a vertically movable platform (having some of the characteristics of an elevator) inside a large pressure vessel, especially a coal-fired boiler-furnace used in generating electricity.
There are many instances in which it is either desirable or necessary for humans to be able to get into a large pressure vessel for the purpose of making a personal examination of things inside the vessel, e.g., the condition of the vessel walls or internal structural members and the like. One of the more common instances in which this kind of examination is necessary can be found in the electrical power industry where large boilers or furnaces are used to generate steam for driving turbines. Such boilers are often 80 to 160 feet high, 50 to 60 feet wide and 60 to 80 feet long. But because such vessels must be pressurized in order to be efficient, the access opening through which inspectors or repairmen must crawl in order to get into such large boilers is frequently on the order of only about 2 feet by 2 feet.
In the prior art, it has been common to pass numerous pieces of relatively small but specially configured sections of steel tubing through a boiler access door, and to then connect that tubing together in such a way as to form what is usually called scaffolding--to establish a rigid framework upon which workers may climb in order to reach elevated spots within the boiler. Regrettably, this technique has not been wholly satisfactory, primarily because it consumes a great number of man-hours and material simply to permit an inspector or repairman to gain access to an elevated location. Probably one reason for the absence of enthusiasm about the technique of using steel scaffolding is that it often takes about two weeks to construct a scaffold inside a large boiler in order to get workmen all the way to the top of a space that is, say, 80 foot tall; a similar two weeks is required to disassemble the scaffold and return the boiler to a clear and operative condition. But even though such portable steel scaffolding has not been favored, it has been used for many years because there was no real alternative that was available.
Another statistic will perhaps be particularly meaningful in evaluating the typical four-week construction time for installing and removing a scaffold in an average boiler, namely, the cost to a utility for having a large boiler out of service. The typical cost for even having a boiler on site and available to generate power is often over $100,000 per megawatt hour--whether the boiler is generating power or not. (This is because of the fixed costs associated with such a large piece of mechanical equipment, including design costs, "operating" salaries, taxes, general overhead, insurance, interest on loans, etc.). Additionally, if a particular utility has one of its units out of service and needs to turn to the national network in order to buy power--to replace the power that it is not able to generate itself, that substitute power is currently purchased at many thousands of dollars per megawatt hour. Very quick and simple arithmetic should make it apparent, therefore, that any kind of a shutdown of a large boiler can quickly become very expensive for a power company.
Besides the economics of wanting to quickly inspect and repair a boiler, there is also the concern for safety of the personnel who must accomplish the work within a typical power plant boiler. Safety for workmen is also a major factor to be taken into account when constructing scaffolds of the prior art, because such boilers typically have a very small floor area that is located at the bottom of a V-shaped region below the burners. And this small floor area is not even completely flat and homogeneous, because it typically includes space for an auger mechanism that is adapted to receive the ash and slag that usually accumulate in the bottom of coal-fired boilers. But while the floor may be only a few feet wide, the area to be encompassed by a work platform at, say, the 80-foot level, is often about 3,500 square feet. The result is that a scaffold must be constructed in such a way that it almost has the appearance of an upside-down pyramid; and it is very difficult to construct a temporary but dependably safe structure within a boiler when the operating parameters are such that the foundation is so much smaller than the superstructure. Perhaps it is for this reason that professional engineers are so reluctant to certify the safety of such temporary structures.
Those who are familiar with the history of art will perhaps recognize that the metal scaffold that has been described above is very similar to the scaffold system that was used by Michelangelo starting in 1508 when he was making preparations to paint the frescoes on the ceiling of the Sistine Chapel. So the concept of using a scaffold-type supporting frame has probably been around for perhaps 500 years or more. When a person learns that maintenance personnel in a modern boiler-furnace are still using a technique that was used by Michelangelo in Rome in the 16th century, one might well ask whether anything else has ever been tried. The answer is "Yes, that several things have indeed been tried, but with no real success." For example, U.S. Pat. No. 4,232,636 to Probazka entitled "Maintenance Platform Built Upon Retractable Support Beams" states that access to the elevated superheater in a boiler-furnace has been attempted by suspending work platforms on cables anchored in the furnace roof. But Probazka reports that such platforms are unsteady and that workers tend to become uncomfortable on them, especially when the platforms are suspended as much as 100 feet above the furnace floor.
Perhaps one reason that the "prior art" platforms described by Probazka were not successful is that maybe they were too much like a freely hanging swing that is suspended from a tree limb, rather than a platform having at least some control features. In any event, the construction proposed by Probazka in his 1980 patent proved to be equally unsatisfactory. A maintenance platform embodying his principles was constructed and tried at a power plant owned by Georgia Power at Plant Bowen, Ga. in 1980; it was rejected by the utility as unworkable when the rollers jammed and the beams could not even be launched to cover the relatively short distance that is illustrated in Probazka's FIG. 1.
But even if the maintenance platform in accordance with the Probazka design could be successfully launched, and if the law of gravity could be negated in such a way that the distal end of a heavy cantilevered beam could be caused to come to rest at the same elevation as the proximal end of such a beam, there are three major problems with such a fixed-level platform. First, such a platform is of necessity restricted to installation in a boiler at some elevation where there is room for storage of the platform when it is not in use. Probazka shows this location to be the "bullnose" of a boiler-furnace. This means that any maintenance work that would have to be done either above or below the bullnose would require the use of either ladders extending above or a traditional boatswain's chair hanging below such a platform. Second, in order to utilize a Probazka platform, at least one wall of a traditional boiler-furnace would have to be substantially modified in order to provide special doors for launching the platform. In particular, the water tubes which normally carry cooling water alongside the furnace walls would have to be eliminated in the spots where the doors are required. The absence of water tubes would contribute to localized hot spots in the walls, and fly ash could be expected to soon accumulate on the inside of the doors, probably to the extent that the doors would effectively become sealed shut from the inside (by the accumulated ash).
Third, the wooden decking shown by Probazka is designed to be installed one piece at a time, after a plurality of horizontal beams have presumably been projected across the narrow "throat" of the boiler-furnace. This may look all right on paper, but modern safety regulations prohibit the introduction of combustible materials (like Probazka's wooden planks) into a pressure vessel where workers may be using heavy-duty construction and repair equipment--like grinders and welders and other things that are likely to generate a spark. The prospect of a welder's torch causing a wooden platform to catch fire when men are in a furnace and working on the platform--and they have only a very limited form of possible escape--should make it quite evident as to why relatively light-weight but dangerous wooden planks have been outlawed by prudent managers.
After a maintenance platform in accordance with the Probazka concept was tried and rejected, a new maintenance platform was designed for use at the same Georgia Power plant, and its use was attempted in 1984. This "improved" platform attempted to overcome the inevitable sagging of the distal end of a cantilevered beam by attaching fixed cables to the projecting beams at their distal ends; the upper ends of the cables were attached to roof anchors. A platform in accordance with this new design was disclosed in U.S. Pat. No. 4,474,497 to Sullivan entitled "Furnace Maintenance Platform." However, a careful study of FIG. 2 of the Sullivan patent will reveal that it suffers from some of the same disadvantages of the Probazka design, namely, the inherent absence of cooling pipes in the regions where doors must be provided in order to launch the cantilevered beams. Also, it should be remembered that the furnaces being represented in the drawing are actually giant pressure vessels. Besides introducing potentially deleterious paths for the leakage of pressurized gases from an operating furnace, these doors are doomed to become sufficiently hot as to attract any hot (and therefore sticky) fly ash that is circulating in the furnace. And the fact that the maintenance platforms must be permanently mounted at a single, fixed location means that a Sullivan platform lacks the versatility that would be desirable for a maintenance platform.
When a platform in accordance with the Sullivan design was found to be unworkable also, still another proposal was made for providing access to the superheater tubes for furnace maintenance. The replacement design is shown in U.S. Pat. No. 4,474,143 to Wincze entitled "Retractable Maintenance Platform Stored Outside of the Furnace." In a Wincze construction the beams were designed to enter the furnace through especially constructed doors in an outwardly bulged furnace wall that could accommodate a beam that is lowered into the furnace with a generally vertical orientation. The theory was that a support cable attached to the distal end of a beam (as shown by cable 42 in Wincze's FIG. 4) could be pulled upward in order to cause the beam to tilt about a specially constructed slot 44 in a support member 46. This concept was tried by Georgia Power in 1984 and was also rejected as being unsuitable for use on the relatively large boiler-furnaces that are now customary in the industry. That is, it was concluded that even if the Wincze concept could be made to work at all, the dead weight of the steel beams and wooden deck in order to span a given work area would probably limit the platform area to a size of about 25 feet.times.25 feet. Such a small size might still be found in an old boiler-furnace having a rating of only about 200 to 300 megawatts; but attempts to improve operating efficiency have now produced modern boiler-furnaces which have generating capacities of three times that amount. Indeed, the average span in a modern boiler-furnace is about 80 feet. It will be seen, therefore, that concepts that have been proposed for old, small boiler-furnaces have been tested in practice and found to be unworkable; and there is no way that a steel platform for bridging a small space could be sized upward to cover the large areas in modern boiler-furnaces. There has therefore remained a need for an improved maintenance platform which is not limited in the area which it may encompass and which does not require any modification to the walls of the boiler-furnace in order to accommodate the platform. It is a primary object of this invention to teach such a new platform.
Another object is to provide at least some of the advantages of a fixed and stable elevator in a building without at the same time introducing those properties of an elevator that would render it unsuitable for boiler or furnace use. For example, U.S. Pat. No. 4,535,727 to Ziegler entitled "Heat Exchanger With Adjustable Platform For Cleaning And Repairing" shows that an elevator-type platform can be permanently placed inside a large pressure vessel. But such a permanent platform is obviously fixed in size and location, and hence can only provide access to the area immediately adjacent the small platform. If an attempt were to be made to transfer this concept from a heat exchanger to a boiler-furnace, and if the platform were to be made large enough to cover the underside of a boiler-furnace superheater (so that workers could have access to the entire superheater for inspection, maintenance or repair), then the presence of the elevator--for only occasional use--would completely negate the normal operation of the boiler-furnace. That is, the combustion of fuel in the lower portion of a boiler-furnace would not be effective in heating water-filled tubes above the furnace region if the elevator platform serves as a physical barrier to the efficient exchange of heat.
Furthermore, a permanently mounted platform as shown by Ziegler could never be accepted for use in the United States in boiler-furnaces, because regulatory agencies like OSHA that are charged with protecting the safety of industrial workers would never authorize it. Such a statement can be confidently made when it is appreciated that any elevator cables that were permanently mounted within a boiler-furnace would be exposed to the direct flames within a furnace and typical heat of perhaps 1700 degrees Fahrenheit; and there is no way that such cables could thereafter be sanctioned as "safe" for supporting humans (or carrying loads above humans) without first completely removing every one of the cables and testing them to make sure that they had not been damaged by the flames or become softened (annealed) from the heat of normal furnace operation. Even a cursory review of safety regulations like those published at 29 CFR XVII Sec. 1910.66, Subpart F should make it abundantly clear as to the amount of concern that regulators have for the safety of suspended platforms. It would obviously be self-defeating to permanently mount an elevator-type device in a boiler-furnace but then be forced to remove all parts of it complete inspection and testing before using the elevator in the manner that was intended. It follows therefore that what is needed is a work and inspection platform that can be safely stored outside a boiler-furnace when the boiler-furnace is being used to generate electricity, etc. On the other hand, such a platform must be capable of being quickly and reliably installed inside a boiler-furnace when such a structure has been shut down and has cooled enough so that it is safe for humans to enter it. It is an object of this invention to provide such a platform.